This invention relates to an inegrated assembly for the grid and cathode of a vacuum tube, and more particularly of a travelling wave tube (TWT).
Vacuum tube amplifiers typically use control grids to control the emission from a cathode. In a typical linear beam tube, for example, as used in military microwave systems (radar, electronic countermeasures, etc.), the cathode is a porous tungsten cylinder or plug. The plug is mounted in a support structure, such as a molybdenum sleeve, with a heater. The cathode plug is machined with a spherical concave end and is impregnated with various compounds which reduce the work function. Then the grids are added. The grids (typically molybdenum) are formed to be concentric with the cathode. They are delicate structures, a net-like configuration with dimensions (thickness) on the order of mils. They are formed by electric discharge machining (EDM) or etching and typically cost on the order of $100 each. The control grid lies close (perhaps 1-3 mils) to the cathode, where it is radiatively heated to about 1000 degrees C. and also subjected to electron bombardment. To reduce the latter, the interception current, a shadow grid is commonly used. The shadow grid is placed on or close to the cathode surface to suppress emission directly next to the control grid. The two grids must be exactly in line and precisely spaced and, or course, electrically isolated. The assembly of the grids is an expensive operation, and typically the yield of acceptable electron guns is low (50%). The assembly is fragile. Often the grids warp out of shape, which effectively ruins a tube which can cost several thousand dollars (some cost $100,000). Attempts to mechanically stabilize the grid locations have included using diamond standoffs between the grids. This failed, because the diamonds deteriorated and became electrically conductive.
United States patents of interest are U.S. Pat. No. 3,651,360 to Sommeria, No. 4,227,116 to Miram et al, No. 4,254,357 to Haas et al and No. 4,405,878 to Oliver. Haas et al are concerned with a multi-arrayed micro-patch electron emitter or cathode having an integral control grid. In FIG. 2 of the patent a control grid 20 is shown spaced from a cathode 12 by an isolator 24 and a high temperature insulator 22. The honeycomb nature of the patented construction is illustrated in FIG. 1. Oliver in FIG. 13 also shows a grid separated from a cathode by layers of material including an insulation layer BN. Sommeria illustrates in FIG. 2 a honeycomb screen grid 10 located on a cathode 1 with a thin support spacing the screen grid from the cathode and thermally isolating the two. Miram et al show in FIG. 2 a concave cathode 20 formed with spaced concave electron emitting depressions 24 and non-emissive grid elements 26 covering the spaces between them. Conductive web elements 28 of a control grid 22 are aligned with the non-emissive shadow grid elements 26 so that small beamlets of electrons are focused through the apertures 29 of grid 22 and miss the conductive web elements 28. However, none of the references suggest a honeycomb ceramic structure for supporting both individual cathodes and a grid. Also of interest is a copending patent application Ser. No. 756,888 filed July 19, 1985 by Amer et al for an "Intergrated Grid Structure", in which the grid assembly comprises two fine photo-etched grids of similar geometry brazed into a sandwich structure in which they remain spaced and electrically isolated from each other by small dielectric posts (spacers) of very small thickness.
Current practice involves a complex and fragile assembly of several parts to form an "integrated grid structure". This is costly, with many process steps, precious metals, etc. The need for grids leads to the problem of how a control grid can be made which is not expensive, which is protected from electron bombardment, which is closely and concentrically spaced from the cathode surface, which is mechanically strong, and which is electrically insulated from the cathode.